On Tue, Aug 23, 2011 at 06:01:00PM +0800, Peter Zijlstra wrote: > On Tue, 2011-08-23 at 11:40 +0800, Wu Fengguang wrote: > > - not a factor at all for updating balanced_rate (whether or not we do (2)) > > well, in this concept: the balanced_rate formula inherently does not > > derive the balanced_rate_(i+1) from balanced_rate_i. Rather it's > > based on the ratelimit executed for the past 200ms: > > > > balanced_rate_(i+1) = task_ratelimit_200ms * bw_ratio > > Ok, this is where it all goes funny.. > > So if you want completely separated feedback loops I would expect If call it feedback loops, then it's a series of independent feedback loops of depth 1. Because each balanced_rate is a fresh estimation dependent solely on - writeout bandwidth - N, the number of dd tasks in the past 200ms. As long as a CONSTANT ratelimit (whatever value it is) is executed in the past 200ms, we can get the same balanced_rate. balanced_rate = CONSTANT_ratelimit * write_bw / dirty_rate The resulted balanced_rate is independent of how large the CONSTANT ratelimit is, because if we start with a doubled CONSTANT ratelimit, we'll see doubled dirty_rate and result in the same balanced_rate. In that manner, balance_rate_(i+1) is not really depending on the value of balance_rate_(i): whatever balance_rate_(i) is, we are going to get the same balance_rate_(i+1) if not considering estimation errors. Note that the estimation errors mainly come from the fluctuations in dirty_rate. That may well be what's already in your mind, just that we disagree about the terms ;) > something like: > > balance_rate_(i+1) = balance_rate_(i) * bw_ratio ; every 200ms > > The former is a complete feedback loop, expressing the new value in the > old value (*) with bw_ratio as feedback parameter; if we throttled too > much, the dirty_rate will have dropped and the bw_ratio will be <1 > causing the balance_rate to drop increasing the dirty_rate, and vice > versa. In principle, the bw_ratio works that way. However since balance_rate_(i) is not the exact _executed_ ratelimit in balance_dirty_pages(). > (*) which is the form I expected and why I thought your primary feedback > loop looked like: rate_(i+1) = rate_(i) * pos_ratio * bw_ratio Because the executed ratelimit was rate_(i) * pos_ratio. > With the above balance_rate is an independent variable that tracks the > write bandwidth. Now possibly you'd want a low-pass filter on that since > your bw_ratio is a bit funny in the head, but that's another story. Yeah. > Then when you use the balance_rate to actually throttle tasks you apply > your secondary control steering the dirty page count, yielding: > > task_rate = balance_rate * pos_ratio Right. Note the above formula is not a derived one, but an original one that later leads to pos_ratio showing up in the calculation of balanced_rate. > > and task_ratelimit_200ms happen to can be estimated from > > > > task_ratelimit_200ms ~= balanced_rate_i * pos_ratio > > > We may alternatively record every task_ratelimit executed in the > > past 200ms and average them all to get task_ratelimit_200ms. In this > > way we take the "superfluous" pos_ratio out of sight :) > > Right, so I'm not at all sure that makes sense, its not immediately > evident that <task_ratelimit> ~= balance_rate * pos_ratio. Nor is it > clear to me why your primary feedback loop uses task_ratelimit_200ms at > all. task_ratelimit is used and hence defined to be (balance_rate * pos_ratio) by balance_dirty_pages(). So this is an original formula: task_ratelimit = balance_rate * pos_ratio task_ratelimit_200ms is also used as an original data source in balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate Then we try to estimate task_ratelimit_200ms by assuming all tasks have been executing the same CONSTANT ratelimit in balance_dirty_pages(). Hence we get task_ratelimit_200ms ~= prev_balance_rate * pos_ratio > > There is fundamentally no dependency between balanced_rate_(i+1) and > > balanced_rate_i/task_ratelimit_200ms: the balanced_rate estimation > > only asks for _whatever_ CONSTANT task ratelimit to be executed for > > 200ms, then it get the balanced rate from the dirty_rate feedback. > > How can there not be a relation between balance_rate_(i+1) and > balance_rate_(i) ? In this manner: even though balance_rate_(i) is somehow used for calculating balance_rate_(i+1), the latter will evaluate to the same value given whatever balance_rate_(i). That is, there is two dependencies, the seemingly dependency in the formula, and the effective dependency in the data values. Thank, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html